Port Differential mail features forward to transactions

Summary:
Ref T2222. Brings the major mail features (affected files, patches) forward.

This drops some of the minor integrations which just show object state (like "Maniphest Tasks") since I think they're not very important. I'll put them back if users miss them.

Test Plan: Sent mail with inline/attached patches.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8459